This invention relates to an apparatus for selectively indexing an aperture wheel along circumferentially spaced points and, more particularly, is concerned with an apparatus that precisely orients apertures formed on the wheel in alignment with a beam of light or similar radiant energy to selectively generate, by photoexposure, different images on a photosensitive surface.
In forming graphic information on a photosensitive surface, line thicknesses inscribed on the surface may be varied according to the pattern of information by selectively positioning different sized apertures in registry with the light beam to generate a single point or spot of light on the photosensitive surface. A line is generated on the photosensitive surface by the relative movement of the light spot and the photosensitive surface. In forming varied detailed graphic information, such as a mask having precise circuitry formed thereon, a photohead must effect successive selections of different sized and/or shaped aperatures rapidly and precisely. Since movement of the photohead relative to the photosensitive surface cannot occur until selection of the proper aperature is complete, a substantial savings in time is realized when the selected aperatures are indexed rapidly.
In U.S. Pat. No. 3,330,182 issued July 11, 1967 an aperature wheel is utilized to selectively position different sized and/or shaped aperatures in registry with a light beam exposing images on a photosensitive surface. A mechanical paw pivots into and out of engagement with circumferentially oriented pins to positively lock the wheel at a selected angular orientation. The accuracy achieved in selectively orienting individual apertures is dependent upon the tolerances maintained between the engaging surfaces of the paw and the pin. Upon continued use, it is conceivable that this accuracy could be diminished with the inherent wear that would accompany the extended use. Moreover, the aperture wheel in this patent cannot rotate to the next desired orientation until the paw pivots out of engagement with the respective pin member. Likewise, the projecting device cannot begin its trace until the paw pivots back into engagement with the newly aligned pin. Thus, the time used in pivoting the paw into and out of engagement with the respective pins can become significant when consecutive indexing is utilized in the formation of complex graphic information.
It is usually desirable to move a photohead at a maximum speed on a coordinate drive system to quickly expose graphic information on the photosensitive surface. The photohead mass should therefore be minimized to reduce the inertial force developed when the photohead is rapidly moved and then stopped by the coordinate drive. Thus, mechanical indexing devices increase the photohead mass and undesirably limit the maximum controllable speed of the coordinate drive since the increased mass generates an additional inertial force that need be compensated by a reduction in the maximum speed.
Accordingly, a general object of the present invention is to provide an improved indexing means for selectively orienting an aperture wheel in a photohead. With more particularity, an object of the present invention is to provide an indexing system for a photohead utilizing an electronic sensor and control means to accurately and swiftly index an aperture wheel located within a photohead at selectively oriented positions.
It is a further object of the present invention to provide a lightweight photohead utilizing an electronic sensor and control means to index a low mass aperture wheel and to therefore eliminate additional mass generated by the use of mechanical indexing elements.
Yet another object of the present invention is to provide a device of the general character mentioned above in which the aperture wheel is quickly rotated to a selected orientation by rotating the wheel in the more favorable of its two rotational directions.
A still further object of the present invention is to provide a photohead utilizing an indexing system having a control means for rotating the aperture wheel at a first velocity until the selected aperture is moved within the vicinity of the light beam and then rotating the aperture wheel at a second slower velocity until the selected aperture is in precise registry with the light beam.
Another object of the present invention is to provide a low cost, lightweight aperture disc having indicators formed integrally with the disc and oriented in association with corresponding apertures.